Seed planters usually include a plurality of seed feeding devices, one for each row being planted. The devices usually include a housing or other means forming a path of travel for seeds to transverse on their way to the ground to be planted. Several types of seed planter monitors utilizing different types of seed sensors in the seed path have been used in the past. One type of seed sensor incorporates mechanical switches which are actuated as a result of physical contact with the seeds, an example of which is described in U.S. Pat. No. 3,632,918. Photoelectric or optical seed sensors which operate by the blocking of a light beam from a light source to a photodetector by the seed have also been used, an example of which is described in U.S. Pat. No. 4,166,948.
In order to have sufficient sensitivity for grain detection, mechanical seed sensors must be designed in such a manner that they are easily deflected by small forces. Consequently, they are sensitive to planter vibration, a problem that is particularly acute for small seeds such as what as wheat or vegetables. Furthermore, the mechanical sensors are designed to have an element interposed in the actual seed path and can consequently block all or part of the seed flow if worn or damaged.
The photoelectric or optical seed sensors are typically designed to operate in a narrowing or constriction of the seed path in order to insure that an individual seed is able to block the light path. This constriction, which "focusses" the seed flow, is subject to blockage by oversize seeds or foreign material, a problem especially important for planters that plant the smaller seeds such as wheat or vegetables. Photoelectric sensors are generally sensitive to sunlight and must be designed to minimize light leakage from the external environment to the photodetector. In addition, photoelectric sensors are prone to reduced sensitivity by the gradual build-up of dust, dirt or deposit from various seed coatings or insecticide or fertilizer on both the light source and the photodetector.
Furthermore, it has heretofore been difficult or impossible to provide a single sensor apparatus capable of detecting different types or sizes of seeds. When the planter machine is changed from one type of seed to another, at least three problems are encountered. First, the difference in seed sizes presents a problem in that a given sensing arrangement having a fixed sensitivity for the purpose of detecting a large seed may fail to respond reliably to a substantially smaller seed. Second, the rate of planting may differ substantially for different typs of seeds such that a sensor having sufficient sensitivity for a relatively slow rate of planting may fail to respond rapidly enough to count all of the seeds being planted at a substantially higher planting rate. Third, the attitude or orientation of the seed may vary substantially when different types of seeds are planted or when different planting rates are used; this is particularly true for asymmetric seeds such as corn, wheat or lettuce.
In the foregoing seed planter monitors, a plurality of indicators are usually provided in the form of lamps corresponding to each seed sensor which flash each time a seed is sensed. This means that the operator must continuously watch the lamps to see whether seeds are being planted. Such arrangements are inconvenient to monitor especially when a large number of rows of seeds are being planted simultaneously since the operator must, among his other functions, continuously watch all the lamps to make sure they are flashing. Furthermore, in case of a malfunction of a single row, if the operator stops the planter, all rows stop planting operation and all lamps will cease to indicate which row was the source of the initial failure; the operator must then either check the planting devices for all the rows or else remember which row was the source of the failure. The task becomes increasingly more difficult as the number of planting rows is increased, for example in the case of wheat planters.